CASUS Institute Seminar, Marcin Kryński Solid State Ionics group, Technical University of Warsaw, Poland
Abstract of the talk// At temperatures just above 1000 K, the pure bismuth oxide in its high temperature cubic δ-phase demonstrates remarkable oxide ion conductivity, reaching approximately 1 S cm-1. However, a phase transition occurs below this threshold, resulting in a sudden decline in conductivity. Extensive scientific endeavors have been directed towards developing bismuth oxide-based compounds capable of maintaining the high conductivity observed in the δ-phase at lower temperatures. Strategies encompass doping techniques or the synthesis of materials where bismuth does not predominate among the cations. In this study, we illustrate how a comprehensive understanding of conduction mechanisms in novel solid electrolytes can be attained through a combination of experimental approaches and theoretical modeling, specifically employing molecular dynamics simulations.
We employ a variety of Density Functional Theory (DFT) methods, with a focus on utilizing versions of the general gradient approximation within molecular dynamics simulations, particularly at high temperatures of up to 1200 K. Depending on the particular aspect being studied, we adapt the traditional ab initio approach by integrating van der Waals interactions or Hubbard corrections as required. All simulations are executed using the Vienna Ab initio Simulation Package. Our calculations typically involve systems ranging from as large as three hundred atoms, particularly when modeling lighter elements, to as small as one hundred atoms for compounds predominantly comprised of heavy ions.
In the rhombohedral Bi1-xPxO1.5 system, characterized by a layered structure we look at a tremendous level of dynamical heterogeneity, with centers of the diffusion processes migrating between the fluorite-like blocks and the van der Waals gap as the Bi/Pr ratio is changed. Within the perovskite sodium bismuth titanate structure, we elucidate the modulation of ionic conductivity attributed to the presence of composition-dependent polarons, which results in the formation of uncommon clusters of oxide ion vacancies. This phenomenon has been leveraged to account for a rapid decline in ionic conductivity observed experimentally with minor alterations in the Bi/Na ratio. In the case of two BIMEVOX Bi2V0.9Ge0.1O5.45 and Bi2V0.95Sn0.05O5.475 compositions a combination of first principle simulations and machine learning methods was used to distinguish between the polar (C2) and nonpolar (C2/m) structures of α-phase, both being considered based on the experimental data.
Marcin will be talking live in Görlitz. However, as the event is organized in a hybrid format that includes a videoconferencing tool by Zoom Inc., people interested in the topic have the chance to also join the talk remotely. Please ask for the login details via contact@casus.science.